DESCRIPTION: A large body of evidence implicates the neuronal growth associated protein GAP-43 in the modulation of neuronal plasticity during both development and learning. However, the molecular mechanisms by which GAP-43 controls these functions are not known. The present proposal uses analyses of functional domains of GAP-43, coupled with studies of its interaction with rabaptin-5, a protein that mediates endocytic membrane fusion, to delineate the molecular events underlying GAP-43 participation in neurite outgrowth ad neurotransmitter release. First, the hypothesis that specific domains of GAP-43 can be linked to specific signaling pathways and to its roles in the distinct processes of neurotransmitter release and neurite outgrowth will be tested. GAP-43 mutants that disrupt functional domains of the protein will be expressed in PC12 cells and neurons lacking GAP-43. The interactions of these mutants with calmodulin, protein kinase C, and the GTP-binding protein G0 will be evaluated, following which the effects of these mutations on neurotransmitter release and process outgrowth will be quantified. The second hypothesis to be tested is that the known interactions of GAP-43 with calmodulin and protein kinase C are tied to its interactions with rabaptin-5 in the presynaptic terminal. Domain mapping, co-localization studies, and quantitative functional assays will clarify the functional role of the binding of GAP-43 to rabaptin-5, and demonstrate whether endocytic membrane fusion events involving these molecules comprise a molecular link between neurotransmitter release and neurite outgrowth. The data obtained from the proposed studies will begin to provide unifying molecular mechanisms for the diverse functions in which GAP-43 is known to participate.